1. Field of the Invention
The present invention relates to a display apparatus and, more particularly, to an output circuit for image information which is suitable for application to a display apparatus using a binary display element such as a ferroelectric liquid crystal display element or the like having a bistability (memory performance) characteristic for an electric field.
2. Related Background Art
Recently, in displays of personal computers, work stations, and the like, realization of a large display screen and a high resolution is rapidly being progressed. Many display modes including the conventional display modes exist. When explaining an example for a graphics environment of a personal computer made of IBM (trade name: International Business Machines Corporation) which is generally frequently used, there are at least ten kinds of display modes such as CGA (Color Graphic Adapter), EGA (Enhanced Graphic Adapter), VGA (Video Graphic Adapter), and the like. The resolutions and the numbers of colors which can be displayed in those display modes are different in accordance with the display mode;
FIGS. 10A-10B show a list of the above modes and their respective characteristics.
(1) With Respect to the Display Color
As will be understood from FIGS. 10A and 10B, the number of constructing bits per pixel (bits/pixel) differs for every display mode. Also storage formats in respective image memories (VRAM) also differ. Apparently, in the mode in which the number of constructing bits per pixel is large, the multi-color display can be executed.
Explanation will now be made as an example with respect to the display mode 13(h) (VGA) which can perform the highest multi-color display in the graphics environment of the personal computer made by IBM Corporation. An output flow of color information is as follows. First, when a certain address in a VRAM is accessed, the image data (bits/pixel: mode 13(h)) in a VRAM functions as an address to select a color register in a color palette in which color information has previously been stored. In the case of the VGA, the color palette has 256 color registers of 18 bits (6 bits for each of R, G, and B). The color information has been stored in the color registers. When one of the 256 color registers is selected by image data from the VRAM, the color data of R, G, and B each comprising six bits are read out and are processed by D/A converters in the same color palette. One D/A converter is provided for each of R, G, and B and converts the 6-bit color data into the analog signal and sends to a display (CRT).
The output method of the color information (color palette + analog output) as mentioned above has advantages such that: the multi-color display can be realized although a data amount of the VRAM is not so large; the color on the display screen can be changed by rewriting the data of the color registers without needing to rewrite the data in the VRAM; the number of lines connected to the display can be reduced; and the like. Therefore, the above method is mainly a standard method in the present personal computers.
(2) With Respect to the Resolution
In FIGS. 10A and 10B, the resolution also differs for every display mode. For instance, the resolution is set to 320.times.200 pixels (picture elements) in the case of the mode D(h) and is set to 640.times.480 pixels in the case of the mode 12(h). Such a method whereby all kinds of display modes are supported by one display (CRT) is hitherto considered to be relatively difficult. In general, the display modes which can display are restricted (limited). On the other hand, in partial CRTs of the automatic tracking type or the like called "multiscan" and "multisync", a method whereby the scanning frequency of an electron beam is switched in accordance with each display mode is used to support the display modes in a relatively wide range. Therefore, if the display is executed in a display mode of small amount of display information (low resolution), the number of characters or numerals which are displayed as rough images is large.
Different from the case of displaying by the CRT or the like, the following points must be considered in the case where various kinds of display modes of different display colors and resolutions are applied to a display apparatus using a liquid crystal, such as a ferroelectric liquid crystal or the like having the memory performance and color information is displayed.
(1) With Respect to the Display Color
In the case of a display apparatus using a binary display element represented by a ferroelectric liquid crystal display apparatus or the like, it is difficult to express gradations (in the depth direction) in an analogwise manner in one pixel (picture element) like a CRT or the like, that is, to three-dimensionally execute a gradation display. In the case of executing the gradation display by the binary display element, in general, a process such that the gradation (color) data in the depth direction is developed in the lateral direction (extending direction) is executed and the color information is two-dimensionally displayed (area gradation). Therefore, in the case where color information is displayed by the ferroelectric liquid crystal display apparatus or the like in various display modes of different display colors, the gradation (color) data in the depth direction which is inherently used for the CRT must be converted into the gradation data in the lateral direction (extending direction) in accordance with the arrangement of the pixels of the actual display apparatus in accordance with the display mode.
(2) With Respect to the Resolution
In the case where the display is executed in various display modes which have conventionally been used in the CRT or the like by using a display apparatus such as a ferroelectric liquid crystal display apparatus or the like of a high resolution (1000.times.1000 pixels or more), the redundant pixels (the pixels remain) occur on the side of the liquid crystal display apparatus because the resolution in the CRT display mode is lower (an amount of display information is smaller) than the effective number of pixels (resolution) of the liquid crystal display apparatus. In such a case, the enlarged display can be also executed by simultaneously driving a plurality of electrodes in the vertical and lateral directions in a lump on the side of the liquid crystal display apparatus. For instance, in the case of displaying the screen in the mode D(h) (320.times.200 pixels) by the ferroelectric liquid crystal display apparatus of 1280.times.1024 pixels or the like, the enlarged display from one time to four times can be realized. Even if such an enlarged display is used, redundant pixels also occur in the portion out of the effective display area depending on the relation between the number of effective pixels (resolution) of the liquid crystal display apparatus and the resolution in the display mode.
Therefore, it is necessary to execute a proper process to the portion (border portion) of the redundant pixels out of the effective display area.
In the case of displaying in the display mode of a low resolution by the CRT, the portion to which an electron beam is not irradiated is maintained black (dark) by thinning out and by scanning the fluorescent display surface by reducing the scanning frequency of the electron beam. However, in the case of the ferroelectric liquid crystal display apparatus, if no image data is input, a state of the pixel is not assured (bright or dark; on or off). Therefore, it is necessary to also input data into the portion of the redundant pixels and to drive and control.